WO 00/58963 (Liquid Audio) discloses that data such as a musical track is saved as a secure portable track (SPT) that can be linked to one or several players and can be linked to a particular saving means, thus restricting reading the SPT to specific players and ensuring that the reading is carried out only from the original saving means. The SPT is linked to a player by the encryption of data of the SPT using a save key that is unique to the player, difficult to change and is guarded by the player under strict security conditions. The SPT is linked to a particular means of saving including data uniquely identifying the save means in a form resistant to falsification, that is, signed in an encrypted manner.
A system for scrambling audio signals is also known from U.S. Pat. No. 4,600,941 (Sony) in which an audio signal is divided into blocks, each of which is formed by a plurality of frames, which plurality of frames is rearranged on a time base in an order predetermined for each block in such a manner as to be encoded. The encoded signal is rearranged on a time base in an original order in such a manner as to be decoded. This system comprises a first circuit for processing the signal to insert a redundant portion into a portion between contiguous frames and to compress the frames in base time in response to the redundant portions during the encoding, a circuit generating a signal for inserting a control signal other than audio information in the redundant portions, a circuit for detecting the control signal for detecting the control signal during decoding and a second circuit for processing the signal for removing the redundant portions in synchronism with the detected control signal and decompressing the frames in base time in response to the redundant portions.
A method and a system for scrambling and descrambling audio information signals is also known from U.S. Pat. No. 5,058,159 (Macrovision corporation). The audio signals are scrambled by inverting the original frequency spectrum in such a manner that the frequency portions that are originally at the bottom in the audio frequency band are shifted to the top, whereas the portions originally at the top of the band are shifted to the bottom. A pilot sound of a known frequency is recorded with the audio signals of the shifted frequencies. During reproduction, each variation in phase and in frequency is searched by its pilot that used to generate the modulation signal for reconstituting the original content in audio signal frequencies.
WO 99/55089 “Multimedia Adaptive Scrambling System” discloses a system for scrambling digital samples representing multimedia data (audio and video) in such a manner that the content of the samples is degraded, but recognizable or otherwise supplied with the required quality. The level of quality is linked to an associated signal/noise ratio and determined with the aid of objective and subjective tests. A given number of LSB's (least significant bits) is scrambled frame by frame in an adaptive manner as a function of the dynamics of the possible values. All the encryption keys are included in the audio/video stream and used by the decoder for descrambling and restoring the stream. After the descrambling the encryption key cannot be recovered because it is scrambled itself by the decoder.
The art provides evidence of a number of systems for protecting audio streams based substantially on the encryption of data, adding encryption keys independent of the content of the audio stream and which therefore modify the format of the structured stream. One particular and different realization is that of the Coding Technologies company, that consists protecting by scrambling a selected part of the bitstream (“bitstream” refers to the binary stream at the output of the audio encoder) and not the entire bitstream. The protected parts represent the spectral values of the audio signal with the result that during the decoding without decryption the audio stream is distorted and disagreeable to the ear.